Oxidative curing printing inks are used for printing documents which must last for a long time and withstand adverse environment conditions, such as is the case for banknotes and identity documents. Oxidative curing is particularly useful in printing processes such as the copperplate intaglio and in the offset printing process, which rely on pasty, oil-based inks of high viscosity (3 Pa*s or more).
The copperplate intaglio printing process, which is traditionally employed for banknote printing, is most often performed with oxidative drying, alkyd-resin based inks. The alkyd groups therein are chosen from the unsaturated fatty acids, derived from linseed oil, tung oil, tall oil, as well as from other drying oils having an iodine number of at least 100, as known to the skilled person (“Drying Oils”: Römpp Lexikon “Lacke and Druckfarben”, 1998, p. 583; Ullmanns Encyclopädie der Technischen Chemie, 4th edition, Vol. 23, p. 425 and following).
The said inks must contain a siccativating agent (drier), which is an oxypolymerization catalyst. After exposure of the printed ink to air, the drier reacts with air oxygen, forming free radicals. These radicals, in turn, initiate crosslinking reactions involving the vicinity of the unsaturated positions (carbon-carbon double-bonds) of the alkyd groups.
Said siccativating agent (drier) is traditionally chosen from the group of metal soaps, comprising the salts of long-chain fatty acids with a polyvalent metal, such as cobalt, manganese, calcium, zirconium, and cerium. Salts of the said type are oil-soluble and thus compatible with the oil-based inks in which they are used.
The siccativating agent must comprise a primary siccativating component, which is a metal ion capable of existing, under the conditions of the application, in more than one oxidation state. The ions of the chemical elements with numbers 23 to 29, as well as those of certain other chemical elements, are potentially useful as primary siccativating components. Manganese soaps display a slow drying action, whereas cobalt soaps are known as rapid driers. Mixtures of both are commonly used for obtaining enhanced drying performance.
Soaps of calcium and/or zirconium, as well as of cerium are often added as co-siccativating agents (secondary siccativating components), to even further accelerate the drying process (see DIN ISO 4619 “Trockenstoffe für Beschichtungsstoffe”).
Metal compounds of alkylenoxide-containing carboxylates, phosphonates and sulphonates, as drying-time-reducing drying agents for oxidatively drying paints, were disclosed in DE 4 236 697 A1 (Henkel KGaA). Cobalt, manganese, iron, lead, vanadium, cerium, zirconium, calcium, strontium, barium, bismuth, zinc and tin were claimed as the metal component of the drying agent.
Soaps of metals such as lead and zinc have been frequently used in the past as components of the drying agent. The increasing consciousness about environmental issues has led, however, to a ban of these metals from applications in consumable goods. Cobalt might also be banned in the future from such applications for the same reasons.
There is in consequence a strong interest in developing cobalt-free drier systems, which can substitute the traditionally used cobalt containing driers in printing inks and coating compositions. None of the cobalt-free drier systems disclosed in the art has, however, reached the performance of the conventional cobalt containing driers in printing inks.
EP-A-1 394 230 (Dainippon Ink and Chemicals, Inc.) discloses a cobalt-free drier for oxypolymerization-drying printing inks, which is based on a mixture of manganese and cerium fatty acid soaps. WO-A-06/03831 of the same applicant discloses a coating composition wherein part of the drier is replaced by a lipoxidase enzyme. WO-A-03/093384 (ATO B.V., Netherland) discloses drier systems based on manganese and/or iron, whose action is accelerated through the addition of a reducing biomolecule, such as ascorbic acid or its derivatives.
A potential substitute for cobalt in drier compositions is vanadium. This element is omnipresent and relatively abundant in the earth crust (136 ppm; 19th most abundant element after zirconium, carbon and sulfur, and before chlorine, chromium and nickel), and is thus not likely to pose a problem if used in consumable goods. This is not the case of cobalt, which is on the one hand an essential trace element (29 ppm; 30th most abundant element), but whose bio-available concentration should, on the other hand, be comprised within well-known limits, any excess of this element being toxic to life. Manganese, on the other hand, is abundant and omnipresent in the earth crust (1060 ppm; 12th most abundant element), such that it can be used without environmental limitations.
Vanadium compounds have been used as oxidation catalysts and are described as siccativating agents for coating compositions. F. H. Rhodes et al., The Journal of Industrial and Engineering Chemistry, Vol. 14, No. 3, p. 222-4 (1922), and R. Swehten, Farben-Zeitung, 32, p. 1138-9 (1927), were the first to report the use of vanadyl resinates and linolates for this purpose. The reported results were challenged, however, by F. Hebler, Farben-Zeitung, 32, p. 2077-8 (1927), showing that the indicated vanadium compounds were clear underperformers, compared to the usual cobalt and manganese drying agents. This statement is confirmed by the historical absence, up to now, of vanadium in commercial siccativating agents.
In recent years, the interest in vanadium driers rose again, and comparative tests performed by R. W. Hein, Double Liaison Physique, Chimie et Economie des Peintures et Adhésifs, No. 492-493, p. 31-2, showed that vanadium soaps can indeed be used as siccativating agents for emulsifiable alkyd resins; the reported comparative drying speeds were as follows: Co/Mn: 5 hours; V: 12 hours; V/Mn: 8 hours; V/Co: 4 hours. The drying was reported to further depend on the presence of drying accelerators, such as bipyridine, which was used in all tests.
Recently, special vanadium compounds were disclosed in U.S. Pat. No. 6,063,841 (Link et al.; Borchers GmbH) as siccativating agents for oxidatively drying, water containing lacquers. The preferred of the said drying agents are aqueous solutions of water-soluble salts of the vanadyl ion (VO2+), e.g. vanadyl carboxylates, in particular vanadyl oxalate, or vanadyl phosphates.
Although the said drying agents are aimed at applications in water-based ink formulations and do not show any drying effect in common oil-based printing inks, we have now surprisingly found that vanadyl (VO2+) salts can under certain conditions nevertheless advantageously replace the cobalt driers in certain types of oil-based printing inks, and that the resulting cobalt-free drier system even outperforms in certain aspects the classical cobalt-manganese based driers.